HARQ protocols have been known as protocols for retransmission control that is used in wireless communication networks.
Transmission and reception of data of the related art, which is performed in accordance with an HARQ protocol, will be described with reference to FIG. 1. The top row of FIG. 1 illustrates a procedure that is performed by a network apparatus NE such as a base station or a high-level node (a radio network controller (RNC)). The bottom row of FIG. 1 illustrates a procedure that is performed by a user terminal UE such as a mobile phone. The network apparatus assigns sub-frames to each of processes, and transmits and receives data. The network apparatus transmits and receives, together with the data, control information such as an identifier identifying the process, an indicator indicating whether or not the data is new data, and so forth.
The processes are HARQ procedures. Units of physical resources are determined for the processes. For example, one time slot or a plurality of time slots are assigned in a time division multiplexing scheme. One sub-carrier or a plurality of sub-carriers are assigned in a frequency division multiplexing scheme. A process number that is used as an identifier for identifying each of the processes is assigned to the process. In HARQ, a plurality of processes independent from one another are simultaneously operated. Physical resources are assigned to as many processes as possible within a round-trip delay time between communication apparatuses. Packets for each of the processes are sequentially transmitted. Thus, a high transmission efficiency is realized.
The indictor is, for example, a new data indicator (NDI). An NDI indicates whether a packet to which the NDI is assigned includes newly transmitted data or retransmitted data. Two values, e.g., one and zero, are used as values of NDIs, and the value of an NDI is changed from one of the two values into the other value every time transmission of data is performed. Accordingly, an apparatus on a receiving side can determine that, when the apparatus has continuously received pieces of data having NDIs whose values are the same, the piece of data transmitted later is retransmitted data.
When the network apparatus NE performs transmission of data for a process denoted by a process number P of zero, the network apparatus NE attaches the process number P of zero, an NDI of zero, a transmittable data size TBsize of A to data 1, and transmits the data 1 using a sub-frame 1.
The network apparatus NE transmits, using a sub-frame 4, data to which another process number is attached for another process. Data 2 that is transmitted using the sub-frame 4 has a process number P of one, an NDI of one, and a TBsize of B. The network apparatus NE changes (toggles) the value of an NDI into a different value every time the network apparatus NE transmits data, thereby indicating that the transmitted data is new data. In an example illustrated in FIG. 1, when transmission of pieces of data is performed using the sub-frames 1, 4, 7, 10, and 13, the value of each of the NDIs is toggled so that the values of the NDIs are sequentially zero, one, zero, one, and zero.
In the sub-frame 7, when the network apparatus NE receives an acknowledgement (ACK) for the data 1 that the network apparatus NE transmitted using the sub-frame 1, the network apparatus NE transmits the next data for the process denoted by the process number P of zero. Furthermore, when the network apparatus NE receives an ACK for the data that the network apparatus NE transmitted using the sub-frame 4, the network apparatus NE transmits the next data for a process denoted by the process number P of one. Thereafter, the network apparatus NE repeats, for each of the process denoted by the process number P of zero (indicated by the solid line) and the process denoted by the process number P of one (indicated by the broken line), a procedure that is similar to the above-described procedure.
When the user terminal UE receives data from the network apparatus NE, the user terminal UE stores the data, and performs cyclic redundancy check (CRC) on the data. When no error is detected, the user terminal UE returns an ACK.
A retransmission procedure of the related art, which is performed in accordance with an HARQ protocol, will be described with reference to FIG. 2. FIG. 2 differs from FIG. 1 in that the network apparatus NE incorrectly detects, as a negative acknowledgement (NACK), an ACK that is transmitted from the user terminal UE.
Because the network apparatus NE detects an NACK for the data 1 that was transmitted using the sub-frame 1, the network apparatus NE retransmits the data 1.
Because the user terminal UE detected no error for the data 1 that was received using the sub-frame 1, the user terminal UE transmitted an ACK back. However, the user terminal UE needs to handle, as new data, the data 1 that the network apparatus NE has retransmitted. In this case, the user terminal UE needs to perform, again, the procedure of storing the data 1, which has been retransmitted, in a buffer, of performing CRC on the data 1, and of returning an ACK when no error is detected.
Also when the network apparatus NE cannot detect an ACK that the user terminal UE has transmitted, a procedure that is similar to the procedure illustrated in FIG. 2 is performed.
The following documents describe related technologies.    Japanese Laid-Open Patent Publication No. 2007-502558.    Japanese Laid-Open Patent Publication No. 2008-092378.    Japanese Laid-Open Patent Publication No. 2006-033156.